Just months after the closure of the Chicago World's Fair, librarian Lydia Bancroft finds herself fascinated by a mysterious dark-haired and dark-eyed patron. He has never given her his name; he actually never speaks to a single person. All she knows about him is that he loves books as much as she does.Only when he rescues her in the lobby of the Hartman Hotel does she discover that his name is Sebastian Marks. She also discovers that he lives at the top of the prestigious hotel and that most everyone in Chicago is intrigued by him. Lydia and Sebastian form a fragile friendship, but when she discovers that Mr. Marks isn't merely a very wealthy gentleman but also the proprietor of an infamous saloon and gambling club, she is shocked. Lydia insists on visiting the club one fateful night and is suddenly a suspect in a murder.

When librarian Lydia Bancroft is rescued by a man who she recogizes as a mysterious book-loving patron of her library named Sebastian Marks, they begin to form a promising friendship, until a shocking truth about Sebastian comes to light.